<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var sum = 100;
        do {
            var op = prompt('请输入您想要的操作\n1.存钱\n2.取钱\n3.显示余额\n4.退出');

            switch (op) {
                case '1':
                    var money = prompt('请输入存入的钱数');
                    sum += parseFloat(money);
                    alert('您现在的余额是' + sum + '元');
                    break;
                case '2':
                    var money = prompt('请输入取出的钱数');
                    sum -= money;
                    alert('您现在的余额是' + sum + '元');
                    break;
                case '3':
                    alert('您现在的余额是' + sum + '元');
                    break;
                case '4':
                    alert('正在退出......');
                    break;

            }
        } while (op != 4)
    </script>
</head>

<body>

</body>

</html>